1. Field of the Invention
The present invention relates to a halftone mask and a method of manufacturing a pattern substrate using the halftone mask.
2. Description of the Related Art
In a manufacturing process of a liquid crystal display, an optical lithography technology is used to form a pattern on a glass substrate, for example. In the optical lithography, a pattern can be obtained by transferring a pattern of a photomask on the substrate coated with a photosensitive resin (hereinafter referred to as a resist) and developing with an alkali developer.
Usually, for a photomask, a pattern using a light blocking film containing chromium or the like is formed over a transparent substrate. The portion with no pattern is where a light transmits the transparent substrate and exposed to be an exposed area. Furthermore, the portion with the pattern is to be a non-exposed area where light is blocked. Then a pattern corresponding to this area is transferred to a resist. Such photomask is generally referred to as a binary mask.
Conventionally in a TFT array substrate manufacturing process of a liquid crystal display, at least 5 etching processes were required. Different masks are needed to each of the etching processes. Therefore, in the conventional process, 5 masks are required. However in recent years, a technique for reducing the number of photomasks required in manufacture to 4 is advancing. By reducing the number of masks, it is possible to reduce the optical lithography processes. Accordingly, a method for performing 2 different etchings with one mask has been considered. More specifically, by providing a difference in the film thickness of the resist intentionally, different etchings are performed.
To be more specific, firstly 2 different kinds of layers are etched consecutively using a resist mask having thin and thick portions. After that, pre-selected thin portion of the resist is stripped by ashing. At this time, the thick portion of the resist becomes thinner and remains. Then using the remaining resist pattern, an etching is performed along with the shape. After that, the resist is removed. In order to perform an etching in this way, a film thickness difference corresponding to the pattern must be created in advance to the resist. To provide the resist with the film thickness difference, a middle gradation area must be formed. In the middle gradation area, a thin resist pattern is formed. On the other hand, in the non-exposed area, a thick resist pattern is formed.
It is known to use a gray tone mask to form the middle gradation area. The gray tone mask has a pattern arranged in slits or lattice which is not resolved at an optical lithography so as to control the transmitted amount of light in the portion where the pattern is disposed. The gray tone mask is also referred to as a slit mask. Furthermore, in these days, a halftone mask for forming a pattern for the middle gradation area has been developed. Such technique is disclosed in Japanese Unexamined Patent Application Publication No. 2000-075466, 2003-322949 and 2004-205551. The halftone mask is to form the middle gradation area using the translucent mask.
However, as the gray tone mask (slit mask) controls the transmitted amount of light according to intervals between gaps in the pattern, an extremely high controllability is required for measurement of a micropattern. Furthermore, as the position of the micropattern is determined by repeating an optical simulation or actual exposure test, it takes a long time to design. Furthermore, when using a lithography device of a wavelength 365 nm as a light source, the resolution increases. Therefore, a precision is required for measurement control of the micropattern for the gray tone mask.
On the other hand, for a halftone mask, the middle gradation area is formed with a translucent film. Thus controlling a transmittance of the translucent film facilitates middle gradation control. On the other hand, an optical image contrast is insufficient in the boundary between the middle gradation area and exposed area. Therefore, the resist film may be reduced by the development. Moreover, if the optical image contract is insufficient, a lateral inclination angle in the edge portion of the resist pattern in the middle gradation area located in the boundary between the exposed area is reduced in the patterning of the resist by an optical lithography and becomes about 10 degrees, for example. Then the area occupied by the resist lateral inclination portion increases. Therefore, process of the micropattern is more difficult as compared to when the inclination angle is large at a dry etching. That is, with a small resist lateral inclination angle, an etching control is difficult.